1. Field of Invention
The present invention relates to binary optical compounds and, more specifically, to a preparation of proteorhodopsin that may transition between binary optical states.
2. Description of Prior Art
Proteorhodopsin (PR) is the general name for a class of closely-related photoactive proteins isolated from marine organisms referred to as the γ-proteobacteria, or group SAR86. This class of protein has been shown to be remarkably similar to the related photoactive protein isolated from Halobacterium salinarum, bacteriorhodopsin (BR), sharing a homologous structure, chromophore, and light-driven proton-pumping activity. Like bacteriorhodopsin, proton pumping in the proteorhodopsins results after the all-trans retinal chromophore absorbs light and isomerizes to 13-cis, which in turn stimulates a characteristic response in the protein, referred to as a photocycle. The photocycle is composed of a number of transient protein intermediates, each spectrally and temporally distinct. Although not identical and somewhat slower, the proteorhodopsin photocycle contains analogs to the K, L, M, and N/O intermediates of the bacteriorhodopsin photocycle. The biophysical nature of these intermediates remains uncertain, although similarities to those in bacteriorhodopsin are assumed, especially in light of the level of conserved amino acids. However, comparisons between these two protein families are limited and should be viewed with caution, considering differences in both the native environment and host organism. It is also important to note that at this time all proteorhodopsin is produced recombinantly by expression of the protein in E. coli. As such, the protein's morphology is different from the most commonly used form of bacteriorhodopsin, i.e., purple membrane. The purple membrane is the plasma membrane and protein construct that is isolated directly from the host organism, Halobacterium salinarum. Therefore, BR remains imbedded in its native membrane, whereas proteorhodopsin isolated from E. coli is detergent solubilized, which is advantageous to many device applications.
To date, two primary classes of proteorhodopsins have been found, green and blue, characterized by the region of the visual spectrum containing each protein's respective absorption maximum. The green proteorhodopsins (BAC31A8 and related) were the first to be described, having a resting state absorption maximum at about 520 nm (at pH>8, 540 nm at pH<7). The blue proteorhodopsins (HOT75M1 and related) have a resting state absorption maximum of about 490 nm (high pH, ˜530 nm at low pH). This variation in spectral sensitivity has been strongly correlated to the marine depths at which the host organisms reside, with the blue proteorhodopsins being found at greater depths than the green.
The photoactive protein bacteriorhodopsin has been a candidate for the active element in a wide range of device applications, ranging from memory storage and holographic materials to nondestructive testing and security inks. All of these applications rely on the protein's unique response to light. The basis for the former applications is the protein's ability to exist in multiple states, defined by unique absorption profiles (i.e., photocycle intermediates); the M and O intermediates have become particularly important. A large number of BR-based applications have been described in the literature. One of the most important innovations in bacteriorhodopsin research was the discovery and characterization of a permanent state, denoted as Q. The Q-state is accessed from the BR O-state by exposure to red light (the purported branched photocycle), and has a 9-cis retinal chromophore that is trapped, but unbound, in the protein-binding site. Exposure to blue light restores the BR resting state. The combination of permanence and wide spectral separation between Q (˜380 nm) and the bR resting state (570 nm) makes BR a good candidate for holographic materials and optical memories. However, in the wild-type protein the bR to Q transition takes place with very poor efficiency, mandating the use of high intensity irradiation and prolonged exposures.